This invention relates to a method of controlling supercharging pressure in an internal combustion engine equipped with a supercharger, and more particularly to an improved method of properly controlling the supercharging pressure at acceleration of the engine.
A supercharging pressure control method for internal combustion engines has conventionally been known, e.g. from Japanese Provisional Patent Publication (Kokai) No. 61-175,239, in which a supercharging pressure control system selectively controls a control valve between (i) a first position in which the cross-sectional area of a passage for exhaust gases for rotating a turbine of the supercharger is increased to make the supercharging pressure higher, and (ii) a second position in which the cross-sectional area of the passage is reduced to make the supercharging pressure lower, in dependence on operating conditions of the engine. At acceleration of the engine, the control valve is brought into and held in the first position for a predetermined period of time after acceleration of the engine has been detected, to thereby enhance the responsiveness of the engine.
However, according to the conventional control method, good accelerability of the engine may not be obtained, depending upon the behavior of the supercharging pressure. Specifically, according to the method, acceleration of the engine is detected based upon the amount of intake air and the rate of change of the opening of a throttle valve of the engine, in such a manner that the engine is deemed to be in a condition of acceleration if both the amount of intake air and the throttle valve change rate are higher than respective predetermined values. When the engine is thus determined to be in a condition of acceleration, the control valve is brought into the first position so as to obtain the maximum supercharging effect. However, as shown by the broken line in FIG. 1, when the driver steps on the accelerator pedal in order to accelerate the engine, for example, the opening .theta.th of the throttle valve mechanically connected to the accelerator pedal increases immediately, as at time point t1 in the figure; whereas the amount of intake air increases due to the supercharging effect, but with some time lag. Therefore, the acceleration is detected with delay as at time point t2. This results in a delay in the rise of the supercharging pressure P.sub.BATC and hence degraded accelerability.
Since the predetermined period of time over which the control valve is held in the first position is set in dependence on the rotational speed of the engine as well as the rate of change of the throttle valve opening, it is impossible to effect the supercharging pressure control in response to the actual rate of rise of the supercharging pressure. For example, when the rate of rise is high, overboost of the supercharging pressure can occur, as at the period between time points t3-t4; whereas when the rate of rise is low, the predetermined period of time elapses so that the control valve is brought out of the first position before the supercharging pressure rises to a sufficient level, as at time point t5, resulting in poor responsiveness at the acceleration of the engine.